1. Field of the Invention
The present invention relates to tunable optical filters. In particular, the present invention relates to tunable optical add, drop, and add/drop filters.
2. Description of the Prior Art
Optical fiber communications systems are theoretically capable of extremely high data rates (terabits per second), meaning that many channels of gigabit rate data can theoretically be carried on a fiber, via wavelength division multiplexing. The utility of fiber optic systems has been limited, however, because of the cost and complexity of the electronics required to separate out a specific wavelength channel or channels at every node in the communications system. Currently, optical add/drop filters are used to extract desired frequencies. FIG. 1 (prior art) shows a conventional fixed-wavelength optical add/drop filter system 100, based on a thin-film interference filter 108.
An add/drop filter 100 such as that shown in FIG. 1, which extracts (drops) and reinserts (adds) a fixed wavelength. A multiplexing filter system which drops and adds four fixed wavelengths is many thousands of dollars. In the future, when fibers may carry as many as 256 wavelengths, the cost of an add/drop multiplexor might be $1 million or more.
Since most nodes in add/drop multiplexor will not need to extract all of the wavelengths, much of the hardware will be idle most of the time. Yet configuring an add/drop filter to extract only particular wavelengths precludes any flexibility at the node.
Recent progress in tunable lasers has produced communication quality lasers which are capable of precise tuning across the entire fiber communication band. This removes the expense of providing dedicated lasers for each channel at each node. The obvious companion to the tunable laser would be a tunable filter, which can drop and add any given wavelength on the fiber. In addition, it would be desirable if the tunable filter could also act as a monitor for the tunable laser so that it could be accurately locked to the dropped channel wavelength. Each node would require only as many tunable laser/tunable filter sets as the maximum number of channels to be read at that node.
Unfortunately, the universal, tunable, drop/add filter described above does not exist. Current drop/add multiplexor filters capable of handling the required close wavelength spacing either use dedicated filters for each channel, or use a cascade of band-splitting filters which result in a separate output fiber for each wavelength. Both methods require network designers to either limit the node""s usable wavelengths or use redundant hardware.
Tunable filters of the Fabry-Perot type are available, but it is not currently feasible to achieve the necessary degree of finesse in these filters. If they can tune the entire WDM range, they do not have narrow enough channels, and if they have narrow enough channels, they can only tune over a portion of the required band. For systems which will require 256 channels, finesse of over 250 is required. A Fabry-Perot filter with this kind of finesse would require very uniform and high reflectivity mirrors, and would be very susceptible to environmental effects such as temperature changes and vibration.
FIG. 2a (prior art) shows a 3-stage Lyot filter 200. Lyot filters were invented in 1933, and are known for achieving a high degree of finesse. The finesse of a Lyot filter increases as 2N, where N is the number of filter stages. Lyot filters having finesse of over 250 are easily achievable.
Referring to the example Lyot filter of FIG. 2a and the frequency plot of FIG. 2b, the operation as follows. The first polarizer 202 polarizes input light 210. Stage 1, comprising delay block 204 and a polarizer 202, passes half of the channels, while discarding the other half. So, for example, the area above the line is passed, while the area below the line is absorbed by the polarizer. Stage 2, comprising delay block 206 and a polarizer 202, does the same thing with the light it receives, passing half of those channels and discarding the other half. Stage 3, comprising delay block 208 and a final polarizer 202, passes half of the channels it receives and discards the other half. Thus, the output light is the light passed through all three stages. The output bands are much narrower than the bands passed by the first stage, but are separated by as much as the centers of the bands passed by the first stage. Further stages make the output bands narrower.
Unfortunately, the Lyot filter has several significant downsides. First, it requires polarized light, so half of the light is lost up front. Second, there is no complementary outputxe2x80x94the light removed at each stage is discarded at the polarizers 202. A 9 stage filter results in a loss of more than 4 dB. Thus, Lyot filters have primarily been used in solar studies, where plenty of light is available and high finesse is essential.
The high finesse of a Lyot filter, without its corresponding loss of light and lack of a complimentary output, would be ideal for use in fiber optic system add/drop filters and multiplexors.
A need remains in the art for a precise, tunable, low loss add/drop filter for use in fiber optic communication systems.
It is an object of the present invention to provide a precise, tunable, low loss add/drop filter for use in fiber optic communication systems. This object is accomplished with a cascaded system of tunable filters.
A tunable drop filter according to the present invention has an input port, a drop port, and an output port and includes means for providing an input signal consisting of channels to the input port, a plurality of filter stages connected to the input port, each filter stage operating to selectively transmit either even or odd channels and reflect either odd or even channels respectively, means for providing reflected channels as a pass signal at the output port, and means for providing a transmitted channel at the drop port.
Each filter stage could comprise a fiber Mach Zender interferometer having a selective delay for transmitting the selected channels and a mirror for reflecting channels not transmitted by the fiber Mach Zender interferometer. The means for providing reflected channels as a pass signal at the output port and the means for providing an add signal at the add port such that the add signal follows the reverse path of the drop signal could comprise circulators.
As a feature, each filter stage could include a delay applied to any reflected channels, each delay selected to synchronize the pass signal channels.
As an alternative, each stage could comprise a bulk optics Mach Zender interferometer having a selective delay for transmitting the selected channels and a mirror for reflecting channels not transmitted by the bulk optics Mach Zender interferometer.
Or, each stage could comprise a selective delay block, a polarizing beam splitter adjacent to the delay block for transmitting the selected channels, and a mirror for reflecting channels not transmitted by the polarizing beam splitter.
A tunable add/drop filter according to the present invention has an input port, an output drop port, an input add port and an output pass+add port and includes means for providing an input signal consisting of channels to the input port, a plurality of filter stages connected to the input port, each filter stage operating to selectively transmit either even or odd channels and reflect either odd or even channels respectively, means for providing reflected channels as a pass signal at the output port, means for providing a transmitted channel at the drop port, means for providing an add signal at the add port such that the add signal follows the reverse path of the drop signal; and means for combining the add signal and the pass signal at the pass+add output port.